The construction of heat exchangers often requires that tubes pass through bores in tube sheets. It is often of critical importance that even small amounts of leakage from the secondary side of a tube sheet to the primary side be avoided. The principal, if not the only substantial source of such leaks is the joints between the tubes and the sheets. The forming of leak proof joints is thus a primary consideration in the construction of such heat exchangers.
In the case of heat exchangers of the type used in nuclear power plants, a steel tube sheet might be two feet thick and have as many as 1000 or more tubes passing through it. Water to be confined to the secondary side of the exchanger may contain levels of contamination that are not acceptable on the primary side. Pretesting of joint forming techniques to assure reliable and repeatable results before construction of the exchanger begins is, therefore, imperative in this context. Moreover, it is also important to be able to demonstrate, in advance, the effectiveness and reliability of the technique to be used.
When the exchanger is to be built, the tubes are positioned in the bores in the tube sheet, and expanded by swagging to form tight joints. In accordance with the present state of the art, swagging is accomplished hydraulically, forcing each tube to expand until it diameterically enlarges the hole in which it has been inserted. When the pressure is removed, the hole diameter is again reduced, due to the elasticity of the tube sheet, and the tube is tightly clamped in place.
Care must be taken to determine the optimum parameters for forming a particular series of joints. It can be difficult to evaluate such factors as the surface finish of the holes and the tubes, the optimum configuration of the holes (which may be entirely cylindrical or many include grooves into which the tubes can expand) and the hydraulic pressure required. Pressures of 30,000 psi and more may be used, but it is desirable to avoid complications associated with unnecessarily high pressures that could result in damage to the tubes or the sheet. For example, if excessive pressure is applied the tube sheet could be deformed in the area of an adjacent joint.
An objective of the present invention is to provide a leak detection apparatus and method for use with tube-and-tube-sheet joints, thereby facilitating testing, experimentation and demonstrations with respect to such joints.